In recent years, an inkjet recording apparatus that performs printing by ejecting ink from a print head has been in widespread use as an apparatus that prints data stored in computers. The inkjet recording apparatus used as an image recording apparatus, such as a printer, a facsimile machine, a copier, or a plotter, includes an inkjet head. The inkjet head includes a nozzle for ejecting ink droplets, an ink passage (also referred to as an ejection chamber, a pressure chamber, a pressurized-liquid chamber, or a liquid chamber) communicating with the nozzle, and an energy generating unit for generating energy for pressurizing ink in the ink passage. The energy generating unit is driven to eject ink droplets from the nozzle that is pressurizing ink in the ink passage, whereby an image is recorded.
As an inkjet head used in the inkjet recording apparatus, a piezoelectric inkjet head is known, in which a piezoelectric element is used as the energy generating unit that generates energy for pressurizing ink in the ink passage and which ejects ink droplets by deforming a vibration plate, which forms a wall surface of the ink passage, by the piezoelectric element.
An electrostatic inkjet head is also known, in which a vibration plate, which forms a wall surface of the ink passage, and an electrode are arranged in parallel as the energy generating unit and which ejects ink droplets by changing the internal volume of the ink passage by deforming the vibration plate by electrostatic force generated between the vibration plate and the electrode.
One type of the inkjet recording apparatus is configured to move a print head back and forth relative to a printing medium in a main-scanning direction (a direction perpendicular to the moving direction of the printing medium), and eject ink for a plurality of colors with either forward movement or backward movement to thereby form dots on the printing medium (one-directional printing). Another type of the inkjet recording apparatus is configured to form dots on the printing medium with both of the forward movement and the backward movement in the main-scanning direction in order to improve print speed (bidirectional printing).
As one feature of the inkjet recording apparatus, an inkjet recording head may be formed as a line head that is in the form of a long shape with a length corresponding to the maximum print width of a recording medium and that is fixed to a main body of the apparatus. With this configuration, it is not necessary to move the inkjet recording head in the main-scanning direction and it is possible to perform image formation only by conveying the recording medium in a sub-scanning direction perpendicular to the main-scanning direction. Therefore, high-speed image formation is possible.
In the inkjet recording apparatuses as above, when a plurality of nozzles is arranged at different positions in the main-scanning direction in order to print high-quality images on printing media, ink ejection timing is adjusted so that dots can be formed at respective predetermined positions.
Further, when bidirectional printing is performed, ink ejection timing is adjusted so that dots formed with the forward movement in the main-scanning direction (hereinafter, referred to as forward dots) and dots formed with the backward movement (hereinafter, referred to as backward dots) can be formed at respective predetermined positions. The timing is adjusted by using a predetermined test pattern.
For example, a technology for modifying a recording pattern so that a user can easily judge the pattern by visual contact has been disclosed (see Japanese Patent No. 4296043).
FIG. 14 illustrates a test pattern for performing conventional dot-deviation adjustment. The test pattern is used for adjusting deviation of formation positions between the forward dots and the backward dots in the bidirectional printing. The test pattern shown in FIG. 14 is formed of recording patterns that are formed by arranging a plurality of vertical ruled lines in parallel at predetermined intervals. The recording patterns of the forward dots are printed according to predetermined timing signals. The recording patterns of the backward dots with recording pattern numbers 0, +1, +2, . . . , are printed according to predetermined timing signals that are shifted from the respective predetermined timing signals for the forward dots in a stepwise manner.
The recording patterns of the forward dots and the recording patterns of the backward dots are printed so that they overlap each other at a certain position in the main-scanning direction (in FIG. 14, the recording pattern with the recording pattern number 0). In the recording patterns with the numbers +1 and +2, because a timing for driving the print head for the backward dots is advanced, the backward dots are deviated to one side (right in FIG. 14) where the dots land preceding the recording patterns of the forward dots. In the recording patterns with the numbers −1 and −2, because a timing for driving the print head for the backward dots is gradually delayed, the backward dots are deviated to another side (left in FIG. 14) where the dots land following the recording patterns of the forward dots. A user selects the number of the recording pattern in which positions of the recording patterns optimally match each other (“0” in FIG. 14), and performs adjustment to eject ink at a print-head driving timing corresponding to the recording pattern number.
However, with the conventional test pattern shown in FIG. 14, there is a problem in that it is difficult to distinguish a difference between adjacent recording patterns (e.g., the recording patterns with the numbers 0 and +1 in FIG. 14) and accuracy for adjusting relative deviation of dot formation positions is inadequate.
This problem occurs because the difference between the adjacent recording patterns exactly corresponds to adjustment accuracy needed by the recording apparatus and the difference is usually very small. Recent inkjet recording apparatuses increasingly have high resolution, and high adjustment accuracy is desired accordingly. Therefore, it is increasingly difficult to distinguish a difference between adjacent recording patterns.
In the technology disclosed in Japanese Patent No. 4296043, because the visually distinguishable range is targeted, it is difficult to perform adjustment with accuracy to the extent that cannot be determined by visual contact when the printing accuracy with higher resolution and higher fineness is desired.
There is another problem in that, as shown in FIG. 15, even when a difference between adjacent recording patterns is large enough to be distinguished by visual contact, if one of the patterns is deviated in the + direction and the other one of the patterns is deviated in the − direction by the same amount, it is difficult to determine which recording pattern should be selected.
When bidirectional printing is performed in the inkjet recording apparatus, even a slight deviation of dot formation positions largely affects the image quality. For example, assuming that a print head that moves in the main-scanning direction tends to form dots at positions deviated to the left from designated positions with the forward movement, the print head forms dots at positions deviated to the right from designated positions with the backward movement. In this manner, the inkjet recording apparatus that cannot fully adjust dot formation positions lead to increase in degradation of image quality. More specifically, when formation positions of a plurality of dots are relatively deviated from one another, surface roughness occurs on a formed image, leading to decrease in the image quality. The above problem also occurs when dots are formed with either the forward movement or the backward movement.
The present invention has been made in view of the above, and it is an object of the present invention to enable to adjust deviation of a print position between dots, which are printed and formed at different timings, by an easy determination method and with good accuracy.